More than a Dream—a Renewable Electricity Future

With improved grid flexibility, energy storage, and transmission infrastructure, renewable energy can power the U.S. electric grid.

The 1.5-MW Vestas wind turbine in Gardner, Massachusetts, supplies 100% of the Mount Wachusett Community College's electricity, and returns power to the grid.Photo by Deborah Donovan, SR Consultant

Imagine the United States several decades in the future. Will renewable energy technologies play a dominant role in U.S. power generation? And if this is to be more than a mere academic exercise, what must we do to realize such a future?

The National Renewable Energy Laboratory (NREL) has been at the forefront of seeking answers to these important questions. The lab's Strategic Energy Analysis Center spearheaded a several-year study to evaluate the future of renewable electricity technologies. More than 110 contributors from 35 entities, including national laboratories, industry, universities, and non-governmental organizations, collaborated on this effort. The resulting Renewable Electricity Futures Study, published in 2012, details the input, approach, analyses, and outcomes of this study.

Prior to this study, NREL and others considered scenarios in which renewable energy technologies within regions of the United States could contribute 20% to 35% of the electricity generated for the power grid. This most recent study explored the implications and challenges of dramatically expanding the penetration levels of renewable electricity generation by 2050—with an end goal of renewable technologies supplying 80% of all U.S. electricity demand.

Changes are already occurring in the current U.S. power generation landscape. The surge of shale gas production, for example, is driving a shift toward natural-gas-fired power plants. This ascendency of gas is coupled with many utilities retiring aging coal-fired plants, largely because of the expense of retrofitting them to meet more stringent air-quality standards, such as for mercury and other toxins. However, there is still interest at the local and federal levels in truly clean—not just somewhat cleaner—energy generation technologies. Renewables have the potential to match or even surpass natural gas generation if gas prices rise, the costs of renewable technologies further decline, and climate and environmental issues continue to be a concern.

Taking into account these and other factors, the NREL-led study concluded that renewable generation could play a much more significant role in the U.S. electricity system than previously considered possible. As one might expect, further work is warranted to investigate and develop this path toward realizing a clean-energy-dominated grid.

Unique Challenges with Renewable Electricity Production

When considering renewable electricity generation, the unique characteristics of some renewable resources may challenge how well our nation's electric system operates. Biomass, geothermal, hydropower, solar, and wind resources are available at sufficient levels for use over wide areas; however, renewable resources can also be considered site specific. For example, values of solar energy are best in the sunny southwest, but the solar resource is more than sufficient to generate significant amounts of electricity across most of the U.S.

Even within regions that are well suited to certain types of renewable energy generation, the electrical output may still be variable and uncertain. For instance, solar intensity varies due to atmospheric conditions, cloud movement, and, of course, the setting sun. Terrain, region, and specific meteorological conditions can impact wind velocity. And because the particular resource—the sun or wind—is variable and often uncertain, the electrical output from solar and wind generation technologies is as well. This is why power plant operators typically categorize solar and wind technologies as "variable generation."

Variable generation can significantly impact how a power grid operates. Consider base-load power plants, such as coal-fired and nuclear plants. While they perform most efficiently and cost effectively when they run continually and at a consistent level of output, they can ramp their outputs, but not as quickly as may be needed. Because of this, operational problems can occur if additional fast-response resources such as natural-gas-fired combustion turbines, storage, or even new "smart grid"-enabled demand response are not available. For instance, if clouds suddenly obscure the sun on a solar power plant, or if the wind velocity abruptly drops across a wind farm, other resources need to quickly deploy to match load and supply. Improved forecasting of solar and wind resources helps operators better handle uncertainty when scheduling generation options.

Of course, similar problems occur with conventional generating equipment when it breaks down, requiring other units to fill the shortfall in generation. The electricity system has been designed to handle these sorts of conditions. The main difference between this type of breakdown and the variance that occurs with renewable sources is that with renewables, the changes can occur frequently and across major segments of the system, even if the changes aren't usually as abrupt as a breakdown would be.

Even though additional variability can challenge system operations, the problem can be minimized by increasing supply- and demand-side flexibility options, storage, and new transmission sources.Illustration provided by NREL, Renewable Electricity Futures Study

Photovoltaic (PV) technologies present another challenge because their peak output does not coincide perfectly with peak demand for electricity. During the hot, sunny Southwestern summer, the peak electric output in a PV system occurs around noon when the sun is highest. Unfortunately, when people return home from work, their air-conditioning preferences cause peak demand to begin in late afternoon and through the evening when the setting sun provides little or no PV energy.

This does not mean that solar energy won't work in this area. In fact, concentrating solar power (CSP) systems provide a great solution. While standard photovoltaic systems convert the sun directly into electricity, CSP systems gather the sun's thermal energy, and then use it to generate electricity. If necessary, this thermal energy can be stored until it is needed for power generation, such as during times of low or no sun. In essence, CSP's ability to store energy allows it to shift its peak output to later in the day, which means it better aligns with the demand.

The Renewable Electricity Futures Study analyzed ways to mitigate exactly these sorts of variable generation challenges. To do this, it needed to evaluate scenarios beyond the case of single energy technologies at individual locations. The analysis simulated an integrated network of generation, transmission, and storage that could reliably meet electric needs both geographically and temporally—that is, across the entire United States and for every hour of the day.

Understanding the Implications

Flexibility to Address Peak Demand on Summer Afternoons

Hot summer afternoons present utilities with a real challenge, especially when trying to incorporate variable generation energy sources. Due to the time of day, these sources may be producing less energy than the peak loads would demand. Utilities can meet this challenge and supply firm capacity by pulling power from diverse reserves.

Conventional utilities without renewables face the same challenge of meeting these peak demands. To do so, they use lower capital-cost gas turbines for the few hundred hours per year that they are needed, and then leave them idle the rest of the year.

With the addition of time-of-day pricing options, utilities are able to charge for energy based on when the electricity is needed and how it is generated. End-users such as industrial customers then shift their energy consumption to off-peak periods when energy costs less, which would help offset peak demand. Similarly, electricity needed to recharge plug-in hybrid electric vehicles would typically represent a demand during off-peak hours.

The extensive report highlighted at least five key findings, along with implications that may raise concerns, identify benefits, or both.

1. Renewable electricity generation, combined with a more flexible electric system, is more than adequate to supply 80% of total U.S. electricity generation by 2050. This sort of system could meet the electricity demand for every region of the contiguous United States every hour of the day.

The analysis only considers generation from renewable technologies that are commercially available today.

2. The flexibility of the electric system needs to be increased to accommodate large amounts of variable renewable energy generation. Flexibility refers to the grid's ability to ride smoothly through changes in generation or demand—whether the changes are known and regular, or related to unplanned events such as severe weather or breakdowns. Greater flexibility will enable operators to more readily maintain the required balance between electricity supply and demand, even at high levels of variable renewable generation on the grid.

Better flexibility can be achieved by developing an energy portfolio that contains a range of options on both the demand and supply sides. A demand-side option, for example, may involve the use of more-responsive loads, such as air-conditioning systems that can be switched off automatically by the power provider during times of excessive peak demand.

The supply side can encourage flexibility in several ways, as well. These include flexible conventional generation, grid storage, and new transmission infrastructure—all of which are aspects of improved energy systems integration. In a 2012 NREL-sponsored forum on the nexus of natural gas and renewables, participants agreed that in some markets, the two types of supplies can be collaborative, rather than competitive. In particular, power plants fired by natural gas, rather than coal, can handle the more frequent and sometimes faster ramping that may be required to balance the variable generation of wind and solar technologies.

Energy storage will also help smooth variability. Grid storage can include a conventional technology such as pumped hydro, in which the potential energy of water pumped to a higher level converts to kinetic energy to spin a generator when needed. Electricity can also be stored directly in batteries. And as discussed previously, thermal energy in a CSP system can be stored for later conversion into electricity. Compared to storing electrical energy directly, storing thermal energy is much more efficient. And because this thermal storage can enable greater use of CSP to provide power late in the day or into evening hours, even more photovoltaic generation may be possible during the daytime.

New transmission lines will allow power, such as that generated at Abengoa's Solana Plant, in Arizona, to be sent back to regions of the eastern United States, where the sun has already set, but where end-users still need more electricity.Photo by Dennis Schroeder, NREL

An 80% renewable energy penetration scenario reduces both greenhouse gas emissions and the amount of water consumed for power generation.Illustration provided by NREL, Renewable Electricity Futures Study

Constructing new transmission lines can access high-quality wind and solar resources in remote locations. In addition, increased transmission capacity will allow operators to average variable generation over larger areas, which smoothes the output. For example, low electric output by PV in cloudy areas can be offset by production in more distant sunny regions. Similarly, low output by wind turbines in areas where the wind is temporarily stilled can be offset by generation in windy areas elsewhere. Overall, sharing resources over greater areas will smooth the availability of these variable renewable resources, enabling their greater overall contributions.

3. Renewables providing 80% of the power generated on the grid will result in substantial environmental benefits, greatly reducing greenhouse gas emissions and helping mitigate potential climate change.

Water use is also an important issue related to conventional and renewable power generation. Coal- and natural-gas-fired power plants use substantial quantities of water, whereas solar PV and wind plants use little or no water. A high-penetration renewables scenario will reduce the use of water, which will help sustainably manage this resource—a resource that is relatively scarce in many regions of the United States.

Visualizing the Transition: Capacity, Dispatch, and Transmission from 2010 to 2050

A study such as Renewable Electricity Futures Study involves analyzing massive amounts of data and a host of assumptions to generate voluminous, modeled output. It's easier to envision this output, and its significance, by viewing interactive time/space depictions. See NREL's calculations of the simulated changes between 2010 and 2050:

Watch the transformation of the electric sector—with the gigawatts added each year between 2010 and 2050 for generation by biopower, geothermal, hydropower, concentrating solar power (CSP), photovoltaics (PV), wind, and fossil and nuclear plants.

Follow the energy flow along transmission lines for each hour in 2050. The nodes of the transmission lines are also highlighted with respect to locational marginal price, load or generation, and imports or exports.

4. Because of the abundance and diversity of renewable resources across the U.S., multiple combinations of renewable technologies could supply 80% of the power on the grid.

For example, if constraints on transmission reduce access to high-quality but distant renewable resources, nearby renewable resources can still readily meet demand at very little increase in cost. Similarly, constraints on grid flexibility or on the availability of resources such as biomass can be balanced by the availability of other renewable resources with appropriate characteristics.

5. The direct, incremental cost associated with high renewable generation compares favorably with published cost estimates for other clean-energy scenarios, such as nuclear and low-emissions fossil options.

Continuing to improve the cost and performance of renewable technologies will reduce this incremental cost even further. Clearly, these cost and performance goals are an ongoing, daily focus of the researchers and managers at NREL.

All Aboard

The Renewable Electricity Futures Study shows that 80% of all U.S. electricity demand can be met with currently commercially available renewable energy technologies at the hourly level every day of the year. However, realizing this vision will require the focus of NREL and others to solve a myriad of complex and interrelated issues of energy systems integration. Achieving the targets of the scenarios analyzed in the study will depend on how successful we are at solving these issues. To do so most effectively, we need to continue lowering the cost of renewable energy technologies, while increasing their reliability, expanding transmission, and improving grid integration.

See the full four-volume Renewable Electricity Futures 2050 report for all the details: